Substrates such as wood, starch-based, and other biodegradable products used in interior or exterior applications can be vulnerable to attack by insects, fungi, microbes, and the like. To prevent decay that tends to result from these attacks, such substrates may be treated with preservatives to protect against decay and increase longevity. Historically, one widely used preservative composition is known by the CCA designation. This designation stands for chromated copper arsenate. CCA compositions were widely used to treat wood products, e.g., Southern Yellow Pine, used for decks, fencing, landscape timbers, and the like.
CCA compositions provide excellent protection against decay. However, relatively recently, health and safety concerns have been raised concerning the arsenic and chromium content of these compositions. Consequently, regulatory guidelines caused CCA usage for residential applications to stop on Jan. 1, 2004. As a result, the industry has developed and continues to develop new preservatives as substitutes for CCA compositions. Uncovering effective substitutes that are chromium and arsenic free has been challenging.
One newer class of copper-based preservatives uses a form of complexed copper that is water-soluble. The resultant solutions are considered homogeneous in the sense that the solutions are a single, liquid phase as applied to substrates. In many embodiments, the copper is complexed with complexing agents such as an alkanolamine. Examples of preservatives that contain copper complexes include copper polyaspartic acid, alkaline copper quaternary ammonium salt (also referred to in the industry by terminology such as “alkaline copper quat” or “ACQ”), copper azole, copper boron azole, ammoniacal copper citrate, copper bis(dimethyldithiocarbamate), and copper ethanolamine carbonate. Commonly, all these have a nitrogen base that complexes copper and carbonate ions to stabilize the resultant complex. Preservative compositions incorporating copper complexed with alkanolamine are referred to by the designation copper-amine and currently dominate the preservative market for residential lumber applications.
As a positive, homogeneous preservative solutions tend to uniformly and thoroughly penetrate substrates. Unfortunately, as compared to biodegradable products treated with CCA materials, biodegradable products treated with these newer copper complex-based materials suffer higher copper losses in the field. Due to the water solubility of the complexes, the copper tends to leach more readily from the treated biodegradable products when exposed to rain or other water. The expectation that copper losses will occur due to leaching causes treatments to be made with larger amounts of copper to accommodate these expected losses. This is costly and wasteful. Also, copper solutions tend to be relatively sensitive to pH changes inasmuch as the soluble complexes can precipitate if the pH is too low. This limits formulation flexibility to the use of only alkaline complexing agents, for instance.
Heterogeneous preservative compositions also have been recently developed. In these, the metal biocide has been present in insoluble, particulate form dispersed in a liquid carrier. This dispersion, emulsion, or the like is then used to treat biodegradable substrates. Examples of heterogeneous preservative compositions in the form of dispersions of micronized copper containing particles are described, for example, in U.S. Pat. Publication Nos. 2004/0258767; 2005/0118280; 2005/0249812; 2005/0265893; 2006/0086284; 2006/0112850; and 2006/0147632.
As a positive, the copper containing particles in heterogeneous treatment compositions exhibit excellent retention characteristics and are highly resistant to leaching as compared to soluble, complexed copper. Unfortunately, the insoluble particles tend to reside only in the pores or other interstitial vacancies of substrates and penetrate poorly into cells or the like. This is believed to result in much less bioefficacy than could be obtained by more thorough and uniform substrate penetration. Heterogeneous strategies also suffer from particle size constraints, inasmuch as the effectiveness of the treatment can be compromised if the particles are too large or too small.
Thus, neither heterogeneous or homogeneous treatment strategies are wholly satisfactory. Homogeneous strategies have good bioefficacy, at least initially, but tend to leach too much. Heterogeneous strategies have good retention, but tend to have less bioefficacy than is desired. Some strategies that can achieve these benefits to a greater degree with lesser suffering from the drawbacks would be very desirable.